1. Field of the Invention
The present invention relates to a rotation holding table for rotating and holding a storage disc thereon and a boss thereof.
2. Prior Art
Computers, especially personal computers have become remarkably widespread recently, and a storage media to be used thereby, particularly, the capacity of a storage disc has been high and the kinds of the storage media are increased. There are a magnetic disc, an optical disc (e.g., CD-ROM), an optical magnetic disc (e.g., MO) as the storage disc. The use of the optical disc as the storage disc has increased recently.
Exemplifying an optical disc called a DVD, a discoid resin substrate that is a single plate constituting the DVD is required to have a thickness of 0.6 mm and an outer diameter of 120 mm and an inner diameter of its central hole of 15 mm. If such a discoid resin substrate is formed of a single discoid resin substrate, it is low in mechanical strength and is easily deformable. Accordingly, discoid resin substrates, each having the same thickness (0.6 mm) are bonded to each other. For example, FIG. 6 schematically shows a DVD of an optical disc formed of two discoid resin substrates (a first discoid resin substrate D1 and second discoid resin substrate D2) which are bonded to each other.
The first discoid resin substrate D1 is formed by a transparent polycarbonate resin by e.g., an injection molding machine. Irregular signal pits (signal holes) serving as information signals, such as sound, are transferred to one surface of the first discoid resin substrate D1. A reflection reducing coating D11 (e.g., aluminum film, etc.) is formed on the signal pits, and a protective coating D12 is formed on the reflection reducing coating D11 to prevent the signal surface from being damaged.
The second discoid resin substrate D2 is formed of the transparent polycarbonate resin, etc. which is the same as the first discoid resin substrate D1 and is bonded onto the first discoid resin substrate D1, in which the signals or information is stored, by way of an adhesive R.
The second discoid resin substrate D2 may be directly bonded onto the first discoid resin substrate D1 by way of the adhesive R without forming the protective coating D12 on the first discoid resin substrate D1.
The optical disc D formed by bonding the first and second discoid resin substrates D1 and D2 receives light reflected on the reflection reducing coating D11 a laser beam by a light detector, not shown, to thereby reproduce the signal.
It is needless to say that the optical disc D called the DVD, can employ a second discoid resin substrate D2 in which signals or information is stored.
Since the high density storage disc including the DVD typically does not use the single substrate or plate and frequently uses plural substrates or plates, the first discoid resin substrate D1 and the second discoid resin substrate D2 need to be bonded to each other.
A series of steps are performed to form an integrated optical disc (i.e., an optical disc) by bonding each single plate (see FIG. 7).
(1) a step of placing the first discoid resin substrate D1 on a placing table C.
(2) a step of coating the adhesive R onto the first discoid resin substrate D1.
(3) a step of placing the second discoid resin substrate D2 on the first discoid resin substrate D1 to thereby bond the former onto the latter.
(4) a step of developing the adhesive R which is interposed between both first and second discoid resin substrates D1 and D2.
(5) a step of curing the developed adhesive R.
Briefly explaining the steps set forth above, first in the step (1), the first discoid resin substrate D1 on which the reflection coating and the protective coating are coated is uniformly subjected to a suction and held on the placing table C.
In the step (2) of coating the adhesive R, the adhesive R, e.g., an ultraviolet curing resin, is jetted from a jetting nozzle N while the placing table C on which the first discoid resin substrate D1 is placed is rotated at a low speed.
It is preferable that the adhesive is jetted in a doughnut-shaped track although tracks of the adhesive R to be jetted onto the first discoid resin substrate D1 are differentiated from one another depending on the movement of the jetting nozzle N.
In the step (3), the transparent second discoid resin substrate D2 is placed on the first discoid resin substrate D1 on which the adhesive R is coated.
In the step (4), the adhesive R interposed between the first and second discoid resin substrates D1 and D2 is developed to be uniformly extended between the first and second discoid resin substrates D1 and D2. The development of the adhesive R is performed by rotating the placing table C at a high speed (normally the speed of rotation is several thousand rpm or more for about several seconds) in a state where the first and second discoid resin substrates D1 and D2 are bonded to each other, namely, where the optical disc D is placed on the placing table C. With the rotation of the placing table C, surplus adhesive R which is present between the bonded first and second discoid resin substrates D1 and D2 is scattered outward when the adhesive R is developed, and also the air sealed between the first and second discoid resin substrates D1 and D2 (air bubbles, etc.) is discharged outward.
In the step (5), the adhesive R, e.g., ultraviolet radiation curing resin layer, is cured by being irradiated with ultraviolet radiation in a state where the bonded and integrated first and second discoid resin substrates D1 and D2 are rotated (e.g., at about several tens rpm) or not rotated.
More specifically, a ultraviolet light source L having a reflecting mirror L1 at the back surface thereof emits light, thereby effectively curing the ultraviolet curing resin layer. Meanwhile, the curing step is different depending on the kind of adhesive R to be used, and hence a curing method conforming to the characteristics of the adhesive R is employed.
As mentioned above, the steps for bonding and integrating both substrates are completed.
The developing step among the above steps is very important because the adhesive R is uniformly extended between the two substrates.
However, the adhesive R is not always uniformly developed by the development using a conventional placing table C. The reason is that the adhesive R is jetted in the doughnut-shape, and further the adhesive R is not sufficiently jetted usually until it reaches the central hole of the optical disc D in the sense that the adhesive R is ultimately effectively used.
Accordingly, in the developing step, there is a problem that the adhesive R is not sufficiently extended in the region adjacent to the central hole of the optical disc D when the adhesive R is developed outward owing to the centrifugal force, etc., and hence the optical disc D is short of the development inside thereof, whereby air remains in the optical disc D.
The inventors of the present application ascertained from testing that the adhesive R is nonuniformly scattered from the central hole toward the inside thereof to thereby contaminate the surface adjacent to the central hole of the optical disc D when the adhesive R is jetted to a part adjacent to the central hole. Considering the performance of the optical disc D, if the adhesive R is not uniformly extended to the region of the central hole, the strength of the region of the central hole is remarkably lowered or the strength of the optical disc is nonuniform as a whole when the adhesive R is cured later. In addition to that, there is not normally present a reflection coating on the central portion of the optical disc D, thereby forming a spotted pattern on the central portion, leading to an inferior beauty or spoiling the beauty of the optical disc D.
The storage disc has been diversified recently as set forth above, and there are various standards in the size thereof (dimensions, etc.), for example, there appears a kind of storage disc having an inner diameter of, e.g., 4 mm, which is different from the inner diameter of the 15 mm of the central hole of the present optical disc D.
As far as the storage disc formed of an integrated plate, which is made of two bonded and integrated discoid resin substrates, is concerned, the two substrates must be bonded to each other using the adhesive, which needs the developing step by the rotation of the storage disc.
However, in the method having the developing step which is caused by the rotation of the optical disc, it is difficult to uniformly develop the adhesive at the region adjacent to the central hole of the optical disc, leading to a deterioration in quality. Accordingly, it is a very important key point that the adhesive is thoroughly and uniformly extended to improve the quality of the optical disc. However, the decisive solution has not been proposed so far.